1. Field of the Invention
The present invention relates to a control circuit for a semiconductor laser diode (hereafter denoted as LD), and a method to control the control circuit.
2. Related Prior Art
Typical optical transmitter applied in an optical communication system provides a function of the auto-power control (hereafter denoted as APC) to keep the average optical power and the extinction ratio of the light emitted from the laser diode. The APC loop generally includes a monitor photodiode (hereafter denoted as PD) to detect the output power of the LD, and a controller to adjust the magnitude of the driving current so as to keep the average output power and the extinction ratio of the output light based on the monitored result. The Japanese Patent application published as JP-2005-123414A has disclosed such an APC control system.
The ordinary APC system usually increases the driving current when the monitor PD breaks down to fallen its output. In such a condition, the APC system decides that the optical output power from the LD decreases and increases the driving current so as to recover the output power from the LD, which results in an extraordinarily optical power from the LD and occasionally brings a situation out of the safety standard. The system disclosed in the prior patent described above inserts a current limited comprised of a resistor in a current path to the LD to suppress the excess current. However, it is well known that ordinary LD shows large temperature dependence in its performance and characteristics. Therefore, the driving current necessary to obtain a preset power is small in a low temperature, while, it becomes large in a high temperature. That is, the maximum current determined by the limiter resistance in the low temperature occasionally is less than a current necessary to obtain the present optical power at the high temperature.
The present invention, taking the subject mentioned above, is to provide a control circuit for the LD able to suppress the increase of the driving current even when the monitor PD breaks down, and to provide a method to control the LD.